After running through the tutorial, I decided to write another version of the plug-in with a few options.

This plug-in requires Python. If you have the 'Filters->Python-Fu->Console' menu item you are all set. If not, install Python support for GIMP.
The plugin can be found under Filters>Artistic>Cross-Process. Don't forget to make the plugin executable.

Just getting this up, so the description will be short for now. Double HSV Film Grain was originally a work-around that simulates Photoshop's film grain filter. It has become more than that, though. Simple to use, it has a few features that go beyond simple noise filters. There is the option to either compress the layers down to the original, or keep the three separate working layers to be manipulated individually. There is also an option to limit the working layers to just two noise layers.

This script adds a realistic film grain effect to a black and white or toned image. To install, copy the attached file to GIMP/share/gimp/2.0/scripts. The larger the file you are running it on, the larger the value you should have for Strength. The value for Sharpness depends on your preferences, but I usually start at 1 and go from there. The lower the value, the sharper the noise.

This script simulates the cut-off of the first picture of a film roll of cheap cameras like Lomo. Role model was this picture from flickr. Copy it into the scripts folder from GIMP, you will find it then under Filters -> Generic -> Photo Border.